1. Field of the Invention
The present invention relates to a laser beam stabilization technique, and more particularly to optimizing a laser output beam by automatically adjusting angular and lateral beam positions based on near and far field beam detection information.
2. Discussion of the Related Art
Lasers are currently being used for industrial material processing. Examples of industrial material processing uses include TFT (thin film transistor) annealing for, e.g., flat panel displays such as for notebook computers or automobile status displays, microlithography and microhole drilling for ink jet nozzles and multichip modules. A laser based work station may be used in these cases and others. The work station includes a laser, a beam delivery system, a mask imaging system and a part handling unit. Optical alignment of the complete system is complicated because even slight deviations of the laser beam from an ideal, or most efficient, position results in reduced performance of the system.
Trained personnel are needed to perform very precise alignment techniques. These techniques are time-consuming, taking several hours in some cases depending on the nature of the system and its components. A complete beam delivery system alignment may be necessary, e.g., after maintenance is performed on an excimer laser resonator such as when a mirror of the resonator requires replacement.
A known beam alignment technique for dye laser applications is performed using an assembly for rotating the laser beam. See U.S. Pat. No. 4,514,849 to Witte et al. The rotation is caused by a rotating wedge. Small perturbations in the output power are then resolved by examining orthogonal components of the rotating beam. Adjustments are automatically made based on the nature of the perturbation information generated by the rotating wedge assembly, to maximize the output of the dye laser.
In another technique, a resonator mirror is modulated in one direction at a first frequency and in another direction, substantially orthogonal to the first, at a second frequency. See U.S. Pat. No. 5,033,061 to Hobart et al. A reference signal correlated with the beam profile is separately extracted for each of the two directions of modulation. Signals for each direction are separately demodulated and error signals are generated. Laser beam alignment is controlled based on information gathered from the error signals.
A similar technique measures the intensity of an output beam for several positions of an adjustable resonator mirror. See Japanese Patent Application Public Disclosure No. Sho 58-222585. The resonator mirror is adjustable in two orthogonal directions and is positioned for operation where the output beam intensity profile is optimal.
The techniques disclosed in Hobart et al., Witte et al., and the Japanese application are understood to improve an output laser beam efficiency by detecting a beam component at one location, and analyzing information in two orthogonal directions. In this way, lateral laser beam positioning at the one location and the efficiency of the laser output beam are improved in both of the orthogonal directions.
Another system for automatically aligning a high power laser beam is disclosed in U.S. Pat. No. 4,146,329 to King et al. The system uses a HeNe-laser beam pre-aligned and coaxial with the high power beam. Each coaxial beam is redirected by a two-axis, gimballed turning mirror. An extraction mirror diverts the main beam and allows the HeNe alignment beam to pass through a small hole at its center. A null position sensor detects the alignment beam. Based on information of the position and intensity of the alignment beam, the position of the high power beam is adjusted to optimize its alignment and output efficiency.
U.S. Pat. No. 4,576,480 to Travis also discloses a technique using a HeNe-alignment beam coaxially directed with a main CO.sub.2 -laser beam. In Travis, the periphery of an alignment aperture is scanned, and misalignment is detected when an electrical detection signal is not constant during traversal of the periphery of the aperture. Each of the techniques of King et al. and Travis utilizes a separate alignment beam to improve the performance of a laser system.
Another technique involves measuring a beam diameter at two locations along its optical path. See U.S. Pat. No. 5,069,527 to Johnston, Jr. et al. In Johnston, Jr. et al., a method is disclosed wherein two axis measurements of a beam profile are detected using a spinning aperture having orthogonally oriented 45 degree opposed knife edges. Variations in the intensity of the beam are measured across its diameter at two locations along an optical path of the beam. The spinning aperture is at the second location when it has rotated by 180 degrees from the first location. Thus, the second location is removed from the first location along the optical path of the beam a distance equal to twice the radius of the circular path of the spinning aperture. In each case, a lens for creating an imaged beam waist is used. Johnston, Jr. et al. provides an instrument, separate from the laser system itself, used for monitoring the positional stability of a laser beam. The instrument does not, however, provide a means of controlling and stabilizing an alignment of the beam.
A precise system for optimizing, controlling and stabilizing a laser output beam is needed. This is particularly true today because advances in the industrial material processing techniques described above are necessitating finer and finer precision laser output beams. Not only is lateral positional precision desired, but also angular precision. To achieve the appropriate degree of precision, a beam detection and adjusting apparatus is desired which measures beam position at two optically significantly spaced-apart locations along its optical path. Automatic adjustment of the lateral and/or angular position of the beam is also desired based on information gathered from the two beam location measurements.